Golf ball manufacturing method

ABSTRACT

The invention provides a method of manufacturing golf balls having a core and at least one cover layer which involves injecting a molten cover resin material around a core so as to form a cover layer. The cover resin material is a single resin composition made up of (A) a thermoplastic polyurethane, (B) a polyisocyanate compound and (C) a thermoplastic elastomer other than the thermoplastic polyurethane in a weight ratio (A):(B):(C)=100:{2-50}:{0-50}, and is injected with from 10 to 99% of the total number of isocyanate groups in component B remaining in an unreacted state within the cover resin material. The method of the invention provides a high manufacturability and a good amenability to mass production, and the manufactured gold balls thus obtained have a high rebound and excellent scuff resistance.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 11/507,464filed Aug. 22, 2006. The above-noted application is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball manufacturing method inwhich a specific thermoplastic polyurethane material is used as thecover material and injection-molded around a core. More specifically,the invention relates to a manufacturing method which is capable ofobtaining a golf ball having a high rebound, excellent spincharacteristics and excellent scuff resistance, and having alsoexcellent flow properties of a cover material in injection-molding andmanufacturability.

The use of polyurethane materials as golf ball cover materials hasreceived attention in recent years. Polyurethane materials are broadlydivided, based on the process used to make molded parts therefrom, intothermoset polyurethane materials and thermoplastic polyurethanematerials. Molded parts made of thermoset polyurethane materials can beobtained by mixing under applied heat a urethane prepolymer havingisocyanate end groups with a liquid starting material such as a polyolor a polyamine as the curing agent, then pouring the mixture directlyinto a mold and heating to effect a urethane curing reaction.

Numerous golf balls which use such a thermoset polyurethane materialhave been disclosed in the prior art (e.g., Patent Reference 1: U.S.Pat. No. 5,334,673; Patent Reference 2: U.S. Pat. No. 6,117,024; andPatent Reference 3: U.S. Pat. No. 6,190,268). Methods of moldingthermoset polyurethane materials are described in, for example, PatentReference 4: U.S. Pat. No. 5,006,297; Patent Reference 5: U.S. Pat. No.5,733,428; Patent Reference 6: U.S. Pat. No. 5,888,437; Patent Reference7: U.S. Pat. No. 5,897,884; and Patent Reference 8: U.S. Pat. No.5,947,843.

Because moldings made of thermoset polyurethane materials lackplasticity when heated, the starting materials and molded parts cannotbe recycled. Moreover, given the length of the heating and curing stepand of the cooling step and given the great difficulty of controllingthe molding time owing to the high reactivity of the starting materialsunder heating and their instability, the manufacturability of moldedparts made of thermoset polyurethane materials, when used as specialtymoldings such as golf ball covers (moldings which encase a corematerial), is regarded as inefficient.

By contrast, moldings made of thermoplastic polyurethane materials arenot obtained by directly reacting the starting materials. Instead, alinear polyurethane material synthesized using starting materials and aproduction method which differ somewhat from those for the thermosetpolyurethane materials described above is employed in the moldingoperation. Such a polyurethane material is thermoplastic, andthermoplasticized polyurethane materials have the quality of solidifyingwhen cooled. Such polyurethane materials can thus be molded using aninjection molding machine. Injection molding a thermoplasticpolyurethane material requires a much shorter molding time than themolding time for a thermoset polyurethane material and moreover issuitable for precision molding, making it ideal as a process for moldinggolf ball covers. In addition, thermoplastic polyurethane materials arerecyclable, and are friendly to the global environment. Golf balls madeusing thermoplastic polyurethane materials are disclosed in, forexample, Patent Reference 9: U.S. Pat. No. 3,395,109; Patent Reference10: U.S. Pat. No. 4,248,432; and Patent Reference 11: U.S. Pat. No.4,442,282.

However, golf ball covers made with conventional thermoplasticpolyurethane materials have been unable to satisfy all of the followingproperties in a golf ball: feel on impact, controllability, rebound, andscuff resistance when hit with an iron.

To address this need, Patent Reference 12: JP-A 9-271538 discloses agolf ball cover made using a thermoplastic polyurethane material havinga high resilience. Yet, even this golf ball cover falls short in termsof its scuff resistance when hit with an iron.

Patent Reference 13: JP-A 11-178949 describes a golf ball cover whichhas a relatively good scuff resistance when hit with an iron and iscomposed primarily of the reaction product of a thermoplasticpolyurethane material with an isocyanate compound. In this cover, anisocyanate compound such as a block diisocyanate or an isocyanate dimeris added as an additive to the thermoplastic polyurethane material.Addition is carried out during melt mixing under applied heat using anextruder or during injection molding, with the reaction being effectedduring molding.

However, in the molding of a cover according to JP-A 11-178949 above,the isocyanate compound is hard to handle because it loses its activityin the presence of moisture, thus making it difficult to obtain a stablereaction product. In the case of blocked isocyanates, which are highlyresistant to moisture absorption, the blocking agent that dissociatesunder heating has a strong odor, making it unsuitable for use in moldingcovers. Moreover, when the isocyanate compound is in the form of apowder or a solution, it is difficult to control the amount of additionto the thermoplastic polyurethane material, making control of the golfball cover properties a challenge. Furthermore, owing to melting pointand melt viscosity differences between the thermoplastic polyurethanematerial and the isocyanate compound, slippage arises within the moldingmachine, which sometimes makes thorough kneading impossible to achieve.In this prior art, for the reasons given above, control of the effectsof moisture within the cover material and of the amount of additiveincluded therein has been inadequate, making it impossible to achievegolf ball covers which are fully satisfactory in terms of their scuffresistance-improving effects.

The preferred thermoplastic polyurethane material described in JP-A11-178949 is based on an aliphatic isocyanate. However, thisthermoplastic polyurethane material has a very high reactivity withisocyanate, making the reaction difficult to control. As a result, oneproblem is that gelation tends to arise before the mixture is used ininjection molding, making it impossible to ensure sufficient plasticity.Another problem is that gelation sometimes occurs during the moldingoperation. Yet another problem is that the resin to be recycledsometimes gels, as a result of which it cannot be reclaimed. Theseproblems have made it difficult to put the above technology to practicaluse.

Patent Reference 14: JP-B 58-2063 (U.S. Pat. No. 4,347,338) describes amethod of manufacturing thermoset polyurethane molded parts whichinvolves intimately mixing a compound having two or more isocyanategroups with a thermoplastic resin which does not react with isocyanategroups, blending the resulting mixture with a thermoplastic polyurethanematerial, then furnishing the blend to a molding machine and molding.However, the objects of this art are simply to improve the solventresistance and the resistance to continuous and repeated friction; thepublished specification makes no mention of the use of this moldingmaterial as a golf ball cover material. Accordingly, there continues toexist a desire for a golf ball cover material which can satisfy a numberof properties required of golf balls; i.e., rebound, distance, spincharacteristics, controllability, feel on impact, scuff resistance, cutresistance, and discoloration resistance.

Patent Reference 15: JP-A 2002-336378 discloses a golf ball which uses acover material composed of a thermoplastic polyurethane material and anisocyanate mixture. The cover material is a thermoplastic polyurethanematerial which is recyclable, has a high rebound, and has an excellentscuff resistance. Such a cover material has both a good thermoplasticpolyurethane manufacturability and exhibits physical propertiescomparable with those of thermoset polyurethanes. At the same time, theplasticizing effect of the isocyanate compound enhances the flowproperties of the thermoplastic polyurethane material, thus enabling themanufacturability to be improved as well. However, this excellent artalso has a number of drawbacks, including the generation of scorchedmaterial when the isocyanate mixture is charged directly into themolding machine, and variability in the mixing proportions due to theuse of a dry blending process, which results in a poor uniformity andleads to poor molding stability. In addition, because the relativeproportions within the isocyanate mixture of the isocyanate compound andthe thermoplastic resin which substantially does not react withisocyanate are already decided, it has been difficult to freely selectthe amounts and types of the isocyanate compound and the thermoplasticresin that one wishes to add.

In addition, Patent Reference 16: JP-A 2002-336380 describes a golf ballwherein a material obtained by blending a thermoplastic polyurethanematerial that contains as the polymeric polyol a polyether polyol havingan average molecular weight of at least 1500 and that has a reboundresilience of at least 40% with a specific isocyanate mixture is used asthe cover material. However, this cover material has the same drawbacksas those associated with the art disclosed in aforementioned PatentReference 15; namely, the generation of scorched material when the covermaterial is charged into the molding machine, poor molding stability,and limitations on selecting the amounts and types of isocyanatecompound to be added.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a method ofmanufacturing golf balls having a high rebound and an excellent scuffresistance, and having also excellent flow properties of a covermaterial in injection-molding and an excellent manufacturability.

As a result of extensive investigations, the inventors have discoveredthat when an injection molding process is used to manufacture golf ballshaving one or more cover layer encasing a core, by using a cover resinmaterial composed primarily of a resin mixture obtained by the admixtureof (A) a thermoplastic polyurethane, (B) a polyisocyanate compound and(C) a thermoplastic elastomer other than the thermoplastic polyurethanein a weight ratio (A):(B):(C)=100:{2-50}:{0-50} and injection-moldingthe cover resin material with from 10 to 99% of the total number ofisocyanate groups in component B remaining in an unreacted state, thecover resin material has high flow properties, problems such as resinscorching do not arise when the isocyanate mixture is directly chargedinto the molding machine, and the molding environment is stable.Moreover, the inventors have also found that golf balls having a moldedcover (cover layer) obtained by the above injection molding process areendowed with a high rebound and an excellent scuff resistance.

Accordingly, the invention provides the following golf balls.

-   -   [1] A method of manufacturing golf balls having a core and at        least one cover layer, the method being comprised of injecting a        molten cover resin material around a core so as to form a cover        layer, wherein the cover resin material is a single resin        composition comprising (A) a thermoplastic polyurethane, (B) a        polyisocyanate compound and (C) a thermoplastic elastomer other        than the thermoplastic polyurethane in a weight ratio        (A):(B):(C)=100:{2-50}:{0-50} and is injected with from 10 to        99% of the total number of isocyanate groups in component B        remaining in an unreacted state within the cover resin material.    -   [2] The golf ball manufacturing method of [1] wherein, when the        cover resin material is injected with an injection molding        machine, the unreacted state of the isocyanate groups in        component B is maintained from a cover resin material feeder and        throughout the interior of the injection molding machine by        filling the molding machine with an inert gas or low dew-point        air.    -   [3] The golf ball manufacturing method of [1] wherein the        components of the cover resin material are present in a weight        ratio (A):(B):(C)=100:{2-30}:{8-50}.

[4] The golf ball manufacturing method of [1], wherein the cover resinmaterial has a melt mass flow rate (MFR) at 210° C. of at least 5 g/10min.

-   -   [5] The golf ball manufacturing method of [1], wherein the cover        resin material includes a recycled resin material.    -   [6] A method of manufacturing golf balls which is characterized        in that a golf ball having a cover layer formed thereon is        post-treated for a specific length of time in up to a 150° C.        temperature environment and/or a high-humidity environment.    -   [7] The golf ball manufacturing method of [6], wherein the        treatment environment is selected from one of (i) to (iv) below:        -   (i) under a heat source of up to 120° C.,        -   (ii) in a constant-temperature, constant-humidity chamber at            a relative humidity of at least 50% but not more than 80%,        -   (iii) in warm water at a temperature of not above 80° C.,        -   (iv) in cold water at a temperature of not above 25° C.    -   [8] The golf ball manufacturing method of [7], wherein the        treatment time does not exceed 5 hours.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball manufacturing method of the invention obtains golf ballshaving a core and one or more cover layer by injecting a molten coverresin material around a core so as to mold the cover layer. The methodof the invention is characterized by the use, as the cover resinmaterial, of a single resin composition containing (A) a thermoplasticpolyurethane, (B) a polyisocyanate compound and (C) a thermoplasticelastomer other than the thermoplastic polyurethane in a weight ratio(A):(B):(C)=100:{2-50}:{0-50}.

As noted above, the cover resin material is composed primarily of athermoplastic polyurethane, and includes: (A) a thermoplasticpolyurethane, (B) a polyisocyanate compound and (C) a thermoplasticelastomer other than the thermoplastic polyurethane.

The total weight of above components A and B combined, while not subjectto any particular limitation, is preferably at least 60%, and morepreferably at least 70%, of the overall weight of the cover layer.Components A, B and C used in the invention are each described in detailbelow.

The thermoplastic polyurethane serving as component A has a structurewhich includes soft segments made of a polymeric polyol that is along-chain polyol (polymeric glycol), and hard segments made of a chainextender and a polyisocyanate compound. Here, the long-chain polyol usedas a starting material is not subject to any particular limitation, andmay be any that is used in the prior art relating to thermoplasticpolyurethanes. Exemplary long-chain polyols include polyester polyols,polyether polyols, polycarbonate polyols, polyester polycarbonatepolyols, polyolefin polyols, conjugated diene polymer-based polyols,castor oil-based polyols, silicone-based polyols and vinyl polymer-basedpolyols. These long-chain polyols may be used singly or as combinationsof two or more thereof. Of the long-chain polyols mentioned here,polyether polyols are preferred because they enable the synthesis ofthermoplastic polyurethanes having a high rebound resilience andexcellent low-temperature properties.

Illustrative examples of the above polyether polyol includepoly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol) and poly(methyltetramethylene glycol) obtained by thering-opening polymerization of a cyclic ether. The polyether polyol maybe used singly or as a combination of two or more thereof. Of these,poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) arepreferred.

It is preferable for these long-chain polyols to have a number-averagemolecular weight in a range of 1,500 to 5,000. By using a long-chainpolyol having a number-average molecular weight within this range, golfballs made of a thermoplastic polyurethane composition having excellentproperties such as resilience and manufacturability can be reliablyobtained. The number-average molecular weight of the long-chain polyolis more preferably in a range of 1,700 to 4,000, and even morepreferably in a range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chainpolyol” refers to the number-average molecular weight computed based onthe hydroxyl number measured in accordance with JIS K-1557.

Suitable chain extenders include those used in the prior art relating tothermoplastic polyurethanes. For example, low-molecular-weight compoundswhich have a molecular weight of 400 or less and bear on the moleculetwo or more active hydrogen atoms capable of reacting with isocyanategroups are preferred. Illustrative, non-limiting, examples of the chainextender include 1,4-butylene glycol, 1,2-ethylene glycol,1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Ofthese chain extenders, aliphatic diols having 2 to 12 carbons arepreferred, and 1,4-butylene glycol is especially preferred.

The polyisocyanate compound is not subject to any particular limitation;preferred use may be made of one that is used in the prior art relatingto thermoplastic polyurethanes. Specific examples include one or moreselected from the group consisting of 4,4′-diphenylmethane diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate,tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate,dicyclohexylmethane diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, norbornenediisocyanate, trimethylhexamethylene diisocyanate and dimer aciddiisocyanate. Depending on the type of isocyanate used, the crosslinkingreaction during injection molding may be difficult to control. In thepractice of the invention, to provide a balance between stability at thetime of production and the properties that are manifested, it is mostpreferable to use 4,4′-diphenylmethane diisocyanate, which is anaromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving asabove component A to be a thermoplastic polyurethane synthesized using apolyether polyol as the long-chain polyol, using an aliphatic diol asthe chain extender, and using an aromatic diisocyanate as thepolyisocyanate compound. It is desirable, though not essential, for thepolyether polyol to be a polytetramethylene glycol having anumber-average molecular weight of at least 1,900, for the chainextender to be 1,4-butylene glycol, and for the aromatic diisocyanate tobe 4,4′-diphenylmethane diisocyanate.

The mixing ratio of activated hydrogen atoms to isocyanate groups in theabove thermoplastic polyurethane-forming reaction can be controlledwithin a desirable range so as to make it possible to obtain a golf ballhaving various improved properties, such as rebound, spincharacteristics, scuff resistance and manufacturability. Specifically,in preparing a thermoplastic polyurethane by reacting the abovelong-chain polyol, polyisocyanate compound and chain extender, it isdesirable to use the respective components in proportions such that theamount of isocyanate groups on the polyisocyanate compound per mole ofactive hydrogen atoms on the long-chain polyol and the chain extender isfrom 0.95 to 1.05 moles.

No particular limitation is imposed on the method for preparing thethermoplastic polyurethane used as component A. Production may becarried out by either a prepolymer process or a one-shot process inwhich the long-chain polyol, chain extender and polyisocyanate compoundare used and a known urethane-forming reaction is effected. Of these, aprocess in which melt polymerization is carried out in a substantiallysolvent-free state is preferred. Production by continuous meltpolymerization using a multiple screw extruder is especially preferred.

Illustrative examples of the thermoplastic polyurethane serving ascomponent A include commercial products such as Pandex T8295, PandexT8290, Pandex T8260, Pandex T8295 and Pandex T8290 (all available fromDIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component B, it iscritical that, in at least some of the polyisocyanate compound in thesingle resin composition, all the isocyanate groups on the moleculeremain in an unreacted state. That is, polyisocyanate compound in whichall the isocyanate groups on the molecule are in a completely free statemust be present within the single resin composition, and such apolyisocyanate compound may be present together with polyisocyanatecompound in which some of the isocyanate groups on the molecule are in afree state.

Various types of isocyanates may be employed without particularlimitation as this polyisocyanate compound. Illustrative examplesinclude one or more selected from the group consisting of4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, p-phenylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. Of the above group ofisocyanates, the use of 4,4′-diphenylmethane diisocyanate,dicyclohexylmethane diisocyanate and isophorone diisocyanate ispreferable in terms of the balance between the influence onprocessability of such effects as the rise in viscosity that accompaniesthe reaction with the thermoplastic polyurethane serving as component Aand the physical properties of the resulting golf ball cover material.

Component C, which is a thermoplastic elastomer other than the abovethermoplastic polyurethane, can be effectively used as an ingredient ofthe cover material in the present invention to further enhance thefluidity of the cover resin material and to elevate the variousproperties required of a golf ball cover material, such as resilienceand scuff resistance. This thermoplastic elastomer is exemplified by oneor more thermoplastic elastomer selected from the group consisting ofpolyester elastomers, polyamide elastomers, ionomer resins, styreneblock elastomers, hydrogenated styrene-butadiene rubbers,styrene-ethylene/butylene-ethylene block copolymers and modified formsthereof, ethylene-ethylene/butylene-ethylene block copolymers andmodified forms thereof, styrene-ethylene/butylene-styrene blockcopolymers and modified forms thereof, ABS resins, polyacetals,polyethylenes and nylon resins. The use of a polyester elastomer, apolyamide elastomer or a polyacetal is especially preferred for suchreasons as enhancing the resilience and scuff resistance while retaininga good manufacturability.

In the practice of the invention, it is critical for the weight ratio(A):(B):(C) of the respective components to be 100:{2-50}:{0-50}, andpreferably 100:{2-30}:{8-50}. By suitably setting the proportions ofcomponents A to C within this range, the fluidity of the cover resinmaterial can be increased and the manufacturability improved, inaddition to which the properties of the cover resin material can befurther enhanced and the resilience and scuff resistance increased.

In the practice of the invention, the resin composition is prepared bymixing component A with component B, and additionally mixing alsocomponent C. It is critical to select the mixing conditions such thatsome or all of the isocyanate groups in component B remain in anunreacted state. For example, a measure such as the mixture of thecomponents in an inert gas (e.g., nitrogen) or in a vacuum state must befurnished. The resin composition is then injection-molded around a corewhich has been placed in a mold. To smoothly and easily handle the resincomposition, it is preferable for it to be formed into pellets having alength of 1 to 10 mm and a diameter of 0.5 to 5 mm. Isocyanate groups inan unreacted state remain in these resin pellets; as the resincomposition is being injection-molded around the core, or due topost-treatment such as annealing, the unreacted isocyanate groups reactwith component A or component C to form a crosslinked material.Pelletization of the above-described resin composition is not subject toany particular limitation. For example, pelletization may be carried outusing a twin-screw extruder.

In addition to above components A to C, various additives may beoptionally included in the above cover resin material. Additives thatmay be suitably used include pigments, dispersants, antioxidants, lightstabilizers, ultraviolet absorbers and parting agents.

The melt mass flow rate (MFR) at 210° C. of the resin composition is notsubject to any particular limitation. However, to increase the flowproperties and manufacturability, the MFR is preferably at least 5 g/10min, and more preferably at least 6 g/10 min. Too low a melt mass flowrate reduces the fluidity, which may cause eccentricity during injectionmolding and may also lower the degree of freedom in the moldable coverthickness. The measured value of the melt mass flow rate is obtained inaccordance with JIS-K7210 (1999 edition).

The cover resin material is preferably used in a form obtained by usingany of various known mixing apparatuses, such as a twin-screw extruder,to pelletize a resin composition prepared by the admixture of abovecomponents A to C and any additives. In the practice of the invention,to fully achieve the objects of the invention, advantageous use can bemade of a procedure that involves, either during or following mixture ofthe components A to C and optional additives using any of the variousknown mixing apparatuses, cooling in an environment isolated frommoisture and pelletization so as to form pellets in an incompletelyreacted state.

As noted above, the invention involves injection molding the cover resinmaterial around the core. The injection molding machine used for thispurpose is generally a known injection molding machine such as istypically used for molding golf balls. The molding temperature variesaccording to such factors as the type of thermoplastic polyurethane, butis typically in a range of 150 to 250° C.

In the practice of the invention, golf ball production is carried out ina state where active isocyanate groups remain present within the coverresin material during injection molding. That is, the manufacturingmethod of the invention eliminates, to the extent possible, factorswhich render the isocyanate groups within component B inactive, the aimbeing to manifest, after injection molding, a crosslinking effectsufficient for attaining the various properties required of the golfball. Deactivation of these isocyanate groups is undesirable for thegolf ball manufacturing method because it has an adverse influence onthe properties of the golf ball, in addition to which it may serve as afactor rendering the manufacturability unstable due to unwanted effectssuch as fluctuations in the injection pressure during injection molding.

To prevent deactivation of the isocyanate groups, it is desirable tofill the injection molding machine, from the cover resin material feedarea and throughout the interior of the injection molding machine viaresin paths, with an inert gas such as nitrogen, or with a low dew-pointdry air, as the medium for transporting the resin. By using this means,the moisture which causes deactivation of the isocyanate groups isremoved and the isocyanate groups are maintained in an unreacted state;that is, the reaction of the isocyanate groups does not proceed anddeactivation of the isocyanate groups can be prevented.

In addition, there is a risk of moisture present in trace amountsreacting with isocyanate groups to generate gaseous carbon dioxide or ofthe polyisocyanate compound vaporizing under heating. To prevent suchsituations from arising, a vacuum exhaust system may be installed withinthe mold to be used.

To prevent deactivation of the above isocyanate groups, injectionmolding may be carried out by vacuum treatment or the like in alow-humidity atmosphere. By carrying out injection molding in such alow-humidity environment, reaction by the isocyanate groups before theresin has been charged into the mold interior is kept from proceeding.As a result, polyisocyanate in which the isocyanate groups are presentin a free state is included to some degree in the resin molded part,thus making it possible to increase the golf ball manufacturability andenhance various properties of the manufactured ball, such as the reboundand scuff resistance.

Techniques that could be used to confirm the presence of polyisocyanatecompound in an unreacted state within the cover resin material includethose which involve extraction with a suitable solvent that selectivelydissolves out only the polyisocyanate compound. An example of a simpleand convenient method is one in which confirmation is carried out bysimultaneous thermogravimetric and differential thermal analysis(TG-DTA) measurement in an inert atmosphere. For example, when the resincomposition (cover material) used in the invention is heated in anitrogen atmosphere at a temperature ramp-up rate of 10° C./min, agradual drop in the weight of diphenylmethane diisocyanate can beobserved from about 150° C. On the other hand, in a resin sample inwhich the reaction between the thermoplastic polyurethane material andthe isocyanate mixture has been carried out to completion, a weight dropfrom about 150° C. is not observed, but a weight drop from about 230 to240° C. can be observed.

It is critical that from 10 to 99%, preferably from 50 to 99%, and morepreferably from 70 to 99%, of the total number of isocyanate groups inabove component B be active (unreacted) isocyanate groups.

After the cover resin material has been injection-molded, the propertiesas a golf ball cover can be further improved by carrying outpost-treatment on the outside surface of the molded cover layer so as toinduce the crosslinking reaction to proceed further.

The above-mentioned crosslinking reaction conceivably involves reactionof the remaining isocyanate groups with remaining OH groups in thethermoplastic polyurethane composition to form new urethane linkages, oran addition reaction by remaining isocyanate groups with urethane groupson the thermoplastic polyurethane to form allophanate linkages.Particularly under conditions such as high humidity or in cold or warmwater, there is a possibility for the formation of urea linkages orbiuret linkages. Hence, such conditions are desirable for improving thescuff resistance.

The heat treatment conditions are not subject to any particularlimitation, although temperature treatment can be carried out atpreferably 150° C. or below, more preferably 120° C. or below, and mostpreferably 80° C. or below. The crosslinking efficiency can be increasedby heat treatment in this way. Here, the higher the temperature at whichheat treatment is carried out, the shorter the time in which thecrosslinking reaction can be brought to completion, although excessiveheat treatment is undesirable because it may promote deterioration ofthe cover layer. With the cover resin material used in the presentinvention, because the crosslinking reaction proceeds in the presence ofmoisture and there is a possibility of obtaining effectivecrosslinkages, an elevated temperature is not always necessary.

In cases where the manufactured golf ball is constructed of a core, anintermediate layer encasing the core and a cover encasing theintermediate layer, and the intermediate layer is made of an ionomerresin, if the heat treatment conditions are set at too high atemperature, the temperature may exceed the cluster melting point Ti ofthe ionomer resin, as a result of which the rebound of the golf ball maydecrease. Also, exceeding the melting point Tm of the ionomer resin mayresult in deformation of the intermediate layer. Hence, it is desirableto carry out treatment at a lower temperature than the cluster meltingpoint (Ti).

No particular limitation is imposed on the heat treatment method. Forexample, heat treatment may be carried out in an oven, or by installinga heat source place within the injection-molding operation and havingthe workpieces pass over that place on the production line. Treatment inthe presence of moisture can be carried out by providing aconstant-temperature, constant-humidity chamber or the like having atemperature of up to 80° C. and a relative humidity of at least 50% orinstalling a steam-generating place on the production line, and havingthe workpieces pass over that place on the line. Alternatively, a coldwater tank at a temperature of up to 25° C. or a warm water tank at atemperature of up to 80° C. may be installed on the production line, andthe workpieces subjected to immersion treatment therein. The heattreatment time correlates with the treatment temperature and thus cannotbe strictly specified, although it should be set within a range thatelicits a heat treatment effect. To achieve an acceptable treatmenteffect, the heat treatment time is preferably at least 30 minutes, morepreferably at least 1 hour, and even more preferably at least 2 hours.More than the required heat treatment is undesirable. The heat treatmenttime is preferably not more than 15 hours, more preferably not more than8 hours, and even more preferably not more than 5 hours, but is notlimited within these time ranges. Depending on the presence of moistureand the treatment temperature, as well as the balance with the treatmenttime, there are cases in which an adverse influence is exerted on theresilience and other resin properties of the molded cover. Hence, it isnecessary to suitably select optimal heat treatment conditions in orderto reliably achieve the desired properties.

In the manufacturing method of the invention, the runners that formduring injection molding can be milled and reused. Here, “runners”refers to excess resin that has solidified in the mold channels foruniformly feeding molten resin during injection molding. In theproduction of molded articles made of thermoplastic resin, these runnersare generally pulverized, mixed with the resin and reused. In workingthe invention, when recycled resin material is included in the resin tobe injection molded, crosslinking reactions proceed in the runner resinand the like as time elapses after molding, causing the viscosity torise. Hence, it is desirable to add such runner resin and the like priorto reaction completion. Moreover, during a re-pelletizing operationinvolving the re-use of milled runners as recyclable resin material, itis desirable to provide a measure for preventing the deactivation ofisocyanate groups like that described above.

At least one of the one or more cover layers in the golf ball of theinvention is made of the above-described thermoplastic polyurethanecomposition. The cover layer formed of the above thermoplasticpolyurethane composition has a surface hardness, expressed as thedurometer D hardness, of generally 30 to 90, preferably 35 to 85, morepreferably 40 to 80, and even more preferably 45 to 75. If the surfacehardness of the cover layer is too low, the spin rate when the ball ishit with a driver may increase, possibly shortening the distancetraveled by the ball. On the other hand, if the surface hardness of thecover layer is too high, the feel of the ball on impact may worsen andthe urethane material may have a poor resilience and durability.

“Durometer D hardness” refers herein to the hardness measured with atype D durometer in accordance with JIS K7215.

The above-described cover layer has a rebound resilience of generally atleast 35%, preferably at least 40%, more preferably at least 45%, andeven more preferably at least 47%. Because a thermoplastic polyurethanedoes not inherently have that good a resilience, strict selection of therebound resilience is preferable. If the rebound resilience of the coverlayer is too low, the distance traveled by the golf ball maydramatically decrease. On the other hand, if the rebound resilience ofthe cover layer is too high, the initial velocity on shots of under 100yards requiring control and on putts may be too high and the feel of theball when played may not agree with the golfer. “Rebound resilience”refers herein to the rebound resilience obtained in accordance with JISK7311.

The core used in the inventive manufacturing method is not subject toany particular limitation. For example, various cores that may be usedinclude solid cores for two-pieces balls, solid cores having a pluralityof vulcanized rubber layers, solid cores having a plurality of resinlayers, and thread-wound cores having a rubber thread layer. Noparticular limitation is imposed on the diameter, weight, hardness,constituent materials and other characteristics of the core.

Moreover, in the invention manufacturing method, in cases where the golfball of the invention has a construction that includes an intermediatelayer, no particular limitation is imposed on the hardness, constituentmaterials, thickness and other characteristics of the intermediatelayer. If necessary, a primer layer may be provided to improve adhesionbetween the intermediate layer and the cover.

It is preferable for the cover layer to have a thickness within a rangeof 0.1 to 5.0 mm. The cover layer is not limited to a single layer, andmay be formed so as to have a multilayer construction of two or morelayers. If the cover is formed with a multilayer construction, theoverall thickness of the cover may be set within the foregoing range.

Golf balls obtained according to the manufacturing method of theinvention are preferably formed to a diameter and weight in accordancewith the Rules of Golf, and are generally formed to a diameter of notless than 42.67 mm and a weight of not more than 45.93 g. The diameteris preferably from 42.67 to 42.9 mm. It is suitable for deflection bythe ball when compressed under a load of 980 N (100 kg) to be generallyfrom 2.0 to 4.0 mm, and especially from 2.2 to 3.8 mm.

As explained above, in the golf ball manufacturing method of theinvention, the cover resin material used has high flow properties,increasing the golf ball manufacturability. Golf balls thus manufacturedhave a high rebound and excellent spin characteristics and scuffresistance.

EXAMPLES

The following Examples of the invention and Comparative Examples areprovided by way of illustration and not by way of limitation.

Examples 1 to 5, and Comparative Examples 1 to 3

Core Formulation:

Polybutadiene rubber 100 parts by weight Zinc diacrylate 24.5 parts byweight Zinc oxide 12 parts by weight Dicumyl peroxide 1 part by weightZinc pentachlorothiophenol 1 part by weight

The core material of the above formulation was kneaded, following whichit was molded and vulcanized at 155° C. for 20 minutes, therebyobtaining a 38.5 mm diameter solid core for a two-piece solid golf ball.The polybutadiene rubber used was BR01 produced by JSR Corporation. Theresulting core had a specific gravity of 1.17 g/cm³, a deflection of 3.4mm when compressed under a load of 980 N (100 kg), and an initialvelocity, measured in accordance with the measurement method of the USGA(R&A), of 78.1 m/s.

In Examples 1 to 4 of the invention, the starting materials shown inTable 1 (units: parts by weight) were worked in a twin-screw extruderand under a nitrogen gas atmosphere, thereby giving resin compositionsin which there remained unreacted isocyanate groups. These resincompositions were in the form of pellets having a length of 3 mm and adiameter of 1 to 2 mm.

The solid core was placed within an injection-molding mold and the covermaterial was injection-molded around the core, thereby giving two-piecegolf balls having a 2.1 mm thick cover. Following injection-molding ofthe cover material, post-treatment was administered under the conditionsshown in the table. The resulting golf ball was held at room temperaturefor one week, following which the ball properties were measured.

Samples for measuring the physical properties of the cover were preparedby injection-molding a 2 mm thick sheet, annealing the molded sheet for8 hours at 100° C., then holding the annealed sheet at room temperaturefor one week. The results are shown in Table 1.

In Comparative Example 1, the solid core was placed within aninjection-molding mold and a dry blend of thermoplastic polyurethanepellets with isocyanate mixture pellets was injection-molded around thecore, thereby giving two-piece golf balls having a 2.1 mm thick cover.In Comparative Example 2, only pellets composed entirely ofthermoplastic polyurethane were injection-molded. Subsequent treatmentwas carried out in the same way as described above for the examples ofthe invention.

TABLE 1 Example 1 2 3 4 5 Thermoplastic Polyurethane 1 100 polyurethanePolyurethane 2 100 100 100 100 (pbw) Polyisocyanate compound (pbw) 9 9 99 9 Thermoplastic elastomer (pbw) 15 15 15 15 15 Titanium oxide (pbw)3.5 3.5 3.5 3.5 3.5 Ultramarine blue (pbw) 0.4 0.4 0.4 0.4 0.4Polyethylene wax (pbw) 1.5 1.5 1.5 1.5 1.5 Montan wax (pbw) 0.8 0.8 0.80.8 0.8 MFR (at 210° C.) 7.8 8.0 8.0 8.0 8.0 Cover Surface hardness 5960 60 60 60 properties (D hardness) Rebound 53 52 52 52 52 resilience(%) Post- In cold water yes treatment (2 hours) In cold water yes (18hours) 50° C. warm water yes (2 hours) 50° C. oven yes (2 hours) 50° C.oven yes (18 hours) Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7properties Weight (g) 45.6 45.6 45.6 45.6 45.6 Deflection (mm) 2.3 2.32.3 2.3 2.3 Initial velocity (m/s) 77.1 77.0 77.0 77.0 77.0 Scuff 23° C.5 5 5 5 5 resistance 13° C. 5 5 5 5 5  0° C. 4 4.5 4.5 4.5 4.5Manufacturability good good good good good

TABLE 2 Comparative Example 1 2 3 Thermoplastic polyurethanePolyurethane 2 0 0 100 (pbw) Polyurethane 3 100 100 0 Isocyanate mixture(pbw) 20 — — Polyisocyanate compound (pbw) — — 9 Thermoplastic elastomer(pbw) — — 15 Titanium oxide (pbw) 3.5 3.5 3.5 Ultramarine blue (pbw) 0.40.4 0.4 Polyethylene wax (pbw) 1.5 1.5 1.5 Montan wax (pbw) 0.8 0.8 0.8MFR (at 210° C.) 2.2* 1.8 8.0 Cover Surface hardness (D hardness) 61 5960 properties Rebound resilience (%) 45 44 52 Post- 50° C. warm water(18 hours) no no yes treatment Ball Diameter (mm) 42.7 42.7 42.7properties Weight (g) 45.7 45.7 45.6 Defection (mm) 2.3 2.4 2.3 Initialvelocity (m/s) 77.1 77.0 76.8 Scuff resistance 23° C. 4 2 5 13° C. 4 2 5 0° C. 3 1 4 Manufacturability NG good good *This is a reference value;the cover material prepared was a mixture of different types of pellets.

Polyurethane 1 (Thermoplastic Polyurethane Material)

Pandex T8295, produced by DIC Bayer Polymer, Ltd.

Polyurethane 2 (Thermoplastic Polyurethane Material)

Pandex T8295 and Pandex T8290 were used in a weight ratio of 75/25. Bothare products of DIC Bayer Polymer, Ltd.

Polyurethane 3 (Thermoplastic Polyurethane Material)

Pandex T8260 and Pandex T8295 were used in a weight ratio of 50/50. Bothare products of DIC Bayer Polymer, Ltd.

-   Explanation of Pandex T8295: A MDI-PTMG-type thermoplastic    polyurethane material having a resin hardness of JIS-A97 and a    rebound resilience of 44%.-   Explanation of Pandex T8290: A MDI-PTMG-type thermoplastic    polyurethane material having a resin hardness of JIS-A93 and a    rebound resilience of 52%.-   Explanation of Pandex T8260: A MDI-PTMG-type thermoplastic    polyurethane material having a Durometer D resin hardness of 56 and    a rebound resilience of 45%.

Isocyanate Mixture

Crossnate EM-30 (an isocyanate masterbatch produced by DainichiseikaColor & Chemicals Mfg. Co., Ltd.; 4,4′-diphenylmethane diisocyanatecontent, 30%; the masterbatch base resin was a polyester elastomer).

Polyisocyanate Compound

4,4′-Diphenylmethane diisocyanate

Polyethylene Wax

Sanwax 161P, produced by Sanyo Chemical Industries, Ltd.

Montan Wax

Licowax E, produced by (Clariant Japan) K.K.

Thermoplastic Elastomer

A thermoplastic polyetherester elastomer (Hytrel 4001, produced byDuPont-Toray Co., Ltd.) was used.

Melt Mass Flow Rate (MFR)

The melt flow rate (or melt index) of the material was measured inaccordance with JIS-K7210 (test temperature, 210° C.; test load, 21 N(2.16 kgf)).

Surface Hardness of Cover Layer

The Durometer D hardness was measured in accordance with JIS-K7215.

Rebound Resilience of Cover Material

The rebound resilience was measured in accordance with JIS-K7311.

Ball Deformation (mm)

The deflection when compressed under a load of 980 N (100 kg) wasmeasured.

Ball Initial Velocity (m/s)

Measured in accordance with the measurement method of the USGA (R&A).

Scuff Resistance of Ball

The ball was held at respective temperatures of 23° C., 13° C. and 0° C.Using a swing robot machine, each ball was hit, using a pitching wedgeas the club, at a head speed of 33 m/s, after which damage from theimpact was visually rated according to the following criteria.

-   -   5: No damage whatsoever or substantially free of apparent        damage.    -   4: Slight damage is apparent, but of minimal concern.    -   3: Surface is somewhat frayed.    -   2: Surface is frayed and damaged dimples are apparent.    -   1: Some dimples are completely obliterated.

Ball Manufacturability

-   -   Good: Molding conditions during mass production were stable;        problems such as scorching of resin were infrequent.    -   NG: Molding conditions during mass production were unstable;        high frequency of problems such as resin scorching.

1. A method of manufacturing golf balls which is characterized in that agolf ball having a cover layer formed thereon is post-treated for aspecific length of time in up to a 150° C. temperature environmentand/or a high-humidity environment.
 2. The golf ball manufacturingmethod of claim 1, wherein the treatment environment is selected fromone of (i) to (iv) below: (i) under a heat source of up to 120° C., (ii)in a constant-temperature, constant-humidity chamber at a relativehumidity of at least 50% but not more than 80%, (iii) in hot water at atemperature of not above 80° C., (iv) in water at a temperature of notabove 25° C.
 3. The golf ball manufacturing method of claim 2, whereinthe treatment time does not exceed 5 hours.